Pump control



May 18, 1965 A. PITT ETAL PUMP CONTROL VFilefd March 31, 1961 34 Je J7 3/ TADEUSZ BUDZ/CH 8K' y ATTORNEYS United States Patent O,

Ohio

Filed Mar. 31, 1961, Ser. No. 99,862 2 Claims. (Cl. 10S-162) This invention relates to means for moving a swash plate of a swash plate pump about an axis which is transverse to the axis of rotation of the pump, in opposite directions trom a determined intermediate position of adjustment at which the output of lthe pump is zero, to effect a determined output of the pump in a forward or reverse qw direction.

By the term swash-plate pump is meant a pump of the kind including a driven shaft, a cylinder block having a plurality of boresv aligned with the axis of the shaft and which is secured to the shaft for rotation unison therewith, a piston in each bore extending from one end of the cylinder block, means biasing the piston outwardly of the bores, a plate valve common to each of the bores and arranged at the other end of the cylinder block, the plate valve connecting each bore in sequence to a supply of hydraulic fluid and then to a pressure line as the shaft and cylinder block is rotated through one complete revolution, and a swash plate having an annular cam face arranged concentn'cally of the shaft adjacent the outwardly extending ends of the piston for the annular cam face to be traversed by said ends of the pistons as the cylinder block is rotated, the swash plate being mounted on trunnions for adjustment about an axis transverse to the axis of rotation of the shaft in order that the inclination of the plate may be varied, and thus the length of travel of the pistons within their bores and the quantity of hydraulic iiuid displaced on each reciprocation be varied, the axis of adjustment of the annular cam plate being such that each piston is sequentially moved inwardly of its bore to a maximum extent immediately before the respective cylinders are disconnected from the pressure line. As will be apparent, in such pump constructions the biasing means of the pistons act on each side of the transverse axis to bias the swash plate to an intermediate position of adjustment in which the output of the pump is zero. Also, the reaction imposed on the swash plate during a pumping stroke is in a direction tending to move the swash plate to said intermediate position of adjustment in which the output of the pump is zero. An example of 'such a pump will be found in our co-pending U.S. application No. 16,502, led March 21, 1960, now U.S. Patent No. 3,006,284.

p According to the invention, in a swash plate pump of v the kind including a swash plate which provides an annular cam face for moving a series of pistons axially of their bores as the pistons are traversed around the cam face on a circular path, the pistons being biased towards the 4cam face by springs, the swash plate being journalled for movement about an axis which extends substantially diametrically of the cam face and being biased by said springs of the pistons towards an intermediate position of adjustment about said axis in which the cam face is perpendicular to said circular path and in which the output of the pump is zero; the means for moving the swash plate about said axis in opposite directions from said intermediate position of adjustment against the bias imposed by said springs and the force imposed on the swash plate by the pressure of the fluid being pumped to effect a determined output of the pump includes a rst piston and cylinder arrangement, means for supplyingfluid at a xed constant pressure to said first piston and cylinder arrange- -ment, means connecting one of said first piston and first cylinder to saidswash plate for the swash plate toV be ice moved in one direction about its axis of movement when fluid under pressure is supplied to said rst piston and cylinder arrangement, a second piston and cylinder arrangement, means for supplying fluid at a selectable constant pressure to said second piston and cylinder arrangement, and means connecting one of said second piston and second cylinder to said swash plate for the swash plate to be moved in an opposite direction about its axis of movement when fluid under pressure is supplied to said second piston and cylinder arrangement, the working area of said second piston and cylinder arrangement being such that at a maximum selectable uid pressure it exerts a force on said swash plate in said opposite direction which is greater than the force exerted by the first piston and cylinder arrangement in said one direction at said fixed constant pressure, the swash plate, when the forces exerted by said first and second piston and cylinder arrangements are equal, lying in said intermediate position of adjustment in which the output of the pump 1s zero.

Preferably, the first piston and cylinder arrangement has a working area such that it can exert a force on the swash plate which is at least equal to the sum of the force exerted by the biasing means in an opposing direction and the force exerted by the pistons during the pumping stroke in order that the swash-plate be moved to an extreme position of movement about its axis in one direction, and the second piston and cylinder arrangement has a working area such that it can exert a force on the swash plate in an opposite direction which is at least equal to the sum of the force exerted by the first piston and cylinder arrangement, of the force imposed by the `biasing means of the pistons, and of the force exerted by the pistons during the pumping stroke in order that the swash plate be moved about its .axis to an extreme position of movement in said opposite direction.v In this manner, the output of the pump can be control-led between maximum output in a forward ow direction to maximum output in a reverse ilow direction.

Another object of the invention is to provide a control lvalve for determining the pressure of fluid applied to the second piston and cylinder arrangement.

According to a further feature of the invention, the control valve includes a valve member and an associated valve seat interposed in a fluid supply line to said second piston and cylinder arrangement, a mechanical linkage associated with the valve member and for moving the valve member off said valve seat when the linkage is moved in one direction, means responsive to fluid pressure occurring downstream of said valve member and valve seat when the valve member is displaced from said seat, said means including a member connected to said linkage and adapted to move said linkage in a valve closing direction, a manually operable member adapted to move said linkage in a valve opening direction, and a resilient member interposed between said manually operable member and said linkage and through which force applied to said manually operable member is transmitted to said linkage, the forces applied to said pressure responsive means and said manually operable member acting in opposition forV the fluid pressure occurring downstream of said valve member to beV proportional to the force applied to the manually operable member.

Preferably, the valve of the invention includes a valve body having an internal cavity and having an inlet to said cavity and lanoutlet from said cavity, a valve member and a complementary valve seat associated with said inlet to control the admission ofY uid Aunder pressure into said cavity, and actuating means for moving the valve member off its seating to admit fluid under pressure to said cavity, the actuating means including a push rod acting on the valve member and movable for moving` the valve member Patented, May 18, i965V relatively to its seating, a lever pivoted at one of its ends from the valve casing for movement in a plane which includes the longitudinal axis of the push rod, the lever acting on one end of the push rod at a position intermediate the ends of the lever, a control piston having a Working face exposed to the pressure within the cavity, a connection between the piston and the other end of the lever for the lever to be moved in a valve-closing direction by the piston on an increase in pressure in said cavity, a spring acting on the lever and biasing the lever in a valve opening direction, and means for varying the bias exerted by said spring in said valve opening direction.

According to a further feature of the invention, the position of the lever is controlled by a spool which is arranged at said other end of the lever, the spool having a pair of spaced lands respectively slidably guided in a pair of aligned bores which are arranged with their axis parallel to the longitudinal axis of the push rod, there being a pivotal connection between the lever at its said other end and the spool at a position intermediate the lands, the mutually presented faces of the lands being exposed to the pressure Within the cavity, and a mutually opposed face being exposed to atmosphere for the spool to be pressure balanced. In this modification, the control piston acts on one end of the spool to move it in a valveclosing direction, and the spring is interposed between the other end of the spool and a manually operable control whereby the bias of the spring in a valve opening direction may be varied.

TsurtherY objects and advantages of the invention will become apparent from a study of the following specication, when taken in conjunction with the single ligure of Vthe accompanying drawings, which igure is a schematic view showing the pump control and control valve of the invention.

Referring to the drawings, a portion of the pump casing is indicated at 10, the casing enclosing a pumping mechanism of the kind hereinbefore described. The pumping mechanism includes a cylinder block A, rotatable about axis B-B and having a plurality of bores C therein. Each bore C has a piston D therein which is spring biased by means of springs E to cause the ends F of the pistons E to bear against the annular cam face G of the swash plate.

Journalled within the casing on a stub shaft 11 is a yoke 12, including an annular cam face'G constituting the swash plate of the pump in order that rotary movement of the yoke 12 will rotate the cam face of the swash plate about the aforementioned transverse axis. Conveniently, roller bearings 13 are inserted between the stub shaft 11 and the yoke 12. v The yoke 12 includes a pair of arms 12a, 12b which extend substantially diametrically of the stub shaft 11, and which are provided at their ends with sockets for the reception of push rods 14, 15. The push rods 14, are journalled on pivot pins 16, 17, respectively, which extend transversely of the sockets.

The push rod 14, at its end remote from the yoke 12, is received in a socket of a piston 18, the push rod 14 being journalled on a pivot pin 19 the axis of which is parallel to the axis of the pivot pin 16. In'this way arcuate movement of the push rod 14 due to arcuate movement of the yoke 12 is accommodated without imposing bending stresses on the push rod 14.

The piston 18 is received in a cylinder 2u which is supported from the pump casing 1u in a rigid manner, the piston 18 and cylinder 20 comprising the first piston and cylinder arrangement hereinbefore referred to and hereinafter referred to in the appended claims.

The end of the push rod 15 remote from the yoke 12 is similarly received in a socket of a piston 21 and is journalled on a pivot pin 22, the axis of the pivot pin 22 being parallel to that of the pivot pin 17 to accommodate relative radial movements of ythe yoke and push rod.

Y As will be observed, the pistons 1S and 21goppose each d other through the linkage provided by the push rod 14, the yoke 12 and the push rod 15.

As will be apparent, the pivot pins 16, 17, 19 and 22 could be dispensed with, and the ends of the push rod could seat directly into their associated sockets, in which case the sockets and the ends of the push rod would preferably be formed with complementary spherical or cylindrical surfaces.

The piston 21 is received in a cylinder 23, the piston 21 and cylinder 23 providing the second piston and cylinder arrangement hereinbefore referred to, and hereinafter referred to in the appended claims.

The bore of the cylinder 20 is connected to an outlet 24a of a constant pressure pump 24, for example, a gear pump, by conduits 25a, 25D, 25C, 25d, and 25e in order that the full output of the pump be applied to the working face of the piston 18. Preferably, a pressure relief valve and controllable by-pass 26 is provided in parallel with the pump 24 in order that the-output of the pump be short-circuted from the outlet 24a to an inlet 24h. The effect of the 'oy-pass 26y is that with the pump 24 running and the by-pass 26 open, zero or negligible fluid pressure will be applied to the Working face of the piston 18.

In the drawing the parts are shown in the position they occupy when the by-pass 26 is closed and the pump 24 is operating, the piston 18 under these Vconditions being at an extreme position of its travel in which the yoke 12 and the associated swash plate are moved from an intermediate position, a, to which they are biased by the springs of the pump, to a position b at which the pump is operating at maximum output in one direction, for example, in a forward direction.

The cylinder 23 is connected by a conduit 27 to the control valve of the invention, indicated generally at 28, the control valve serving to control the pressure of fluid supplied to the bore of the cylinder 23 from the pump 24 in a manner hereinafter described.

It will be observed that the working face of the piston 21 is of greater area than the Working face of the piston 18, .and in practice, the ratio of those areas would be in excess of two to one. The effect of this difference in working area is to enable the yoke 12 to be moved to one extreme position of movement b against the bias of the springs and the reaction of the pumping pressure on the swash. plate when the full output of the pump 24 is applied to the piston 18, and to enable the yoke to be moved against the bias of the piston 18, of the springs, and of the reaction of the pumping pressure on the swash plate to its other extreme position of adjustment c when the full output ofthe pump is applied to the working face of the piston 21. In this manner the output of the pump can be controlled Ybetween maximum output in a forward flow direction, through the intermediate position a at which the output of the pump is zero, to maximum output in a reverse ow direction.

Preferably the cavity within the casing 10 is connected by a scavenge conduit 29 to the inlet 21E-b of the pump in order that any hydraulic fluid which may seep past the pistons 1S or 21 may not accumulate within the cavity in a manner causing a hydraulic lock. To accommodate the changing volume of the hydraulic iluid within the system, the inlet 24b of the pump is connected to the usual reservoir by a conduit 30. Y

Referring now more particularly to the control valve 28, that valve includes a body 31 having an internal cavity 32. An inlet to the cavity is provided at 33, the inlet being connected to the outlet 24a of the pump by a conduit 34 'and an outlet 35 is provided from the cavity, the outlet being connected directly to the conduit27.

The ingress of hydraulic uid to the cavity 32 is controlled by a ball valve 35 which opens against Vthe uid flow, and which is pressed onto a seating 37 by a light spring 38. .The spring 33 is for the purpose of returning the ball to its seating after it'has been displaced therefrom. Preferably, a pin 36a is. securedrto the ball and 5. extends within the spring to assist in the centering and seating action.

The valve seat is formed at one end of a threaded plug 39 which extends through a gallery 40, and which is provided with axial and radial bores 41 for the purpose of permitting hydraulic uid to pass into the gallery 4t) and into the cavity 32 when the ball valve 36 is moved off its seating.

The ball valve 36 is moved off its seating by a push rod 42 having an enlarged end 42a which abuts a spherical enlargement 43 intermediate the ends of a lever 44. One end 44a of the lever 44 is formed as a sphere which is received in a spherical recess of a threaded plug 45 in order that the lever may be moved in a plane which includes longitudinal axis of the push rod 42, and the other end 44b of the lever is for-med as a sphere and is received between a pair of discs 46 which are carried by a waisted portion 47 of a spool 48. The spool 4S includes a pair of lands 49, which are slidably received in axially aligned bores 52 of the valve body.

As will be observed, the mutually presented faces of the lands 49 and 50 are exposed to the pressure within the cavity yfor the spools to be pressure balanced, and the mutually opposed lfaces of the lands 49 and 50 are in communication ywith atmosphere through vents S3 and S4.

At its end adjacent the land 50, the spool 48 is provided with an extension 55 which abuts a piston `56 in the form of a short rod. The piston 56 extends through a wall 57 of the valve body into a chamber 58 which is in communication with the gallery 40 by a passage 59. It will thus be seen that the pressure within the cavity 32 is applied to the working yface ofthe piston 56, and acts to move the piston and the spool valve towards the right in the drawings, i.e., in a valve closing direction.

At its end adjacent the land 49, the Ispool 4S is provided with an extension 66 which is engaged by a ydisc 61 providing an abutment for one end of a relatively strong spring 62. The remote end of the spring 62 abuts a similar disc63 which is in direct contact with yan end wall of a bio-re 64a of a manually operable plunger 64, the plunger being slidable within a bore 65 of the valve body 3l, and being located against displacement out of the -bore 65 by a G-ring 66.

The operation of the control valve is as follows:

' With the parts in the position `shown in the drawings, the ball valve 36 is abutting its seating 37, and the cavity 32 is in communication with the inlet 2'4b of the pump 24 through an orifice 67 of a bleed valve 68.

If now the plunger 64 is moved to the Ileft in the drawings, the spring 62, and with it the spool 43 and piston 56 will be moved to the left in the drawings, thus causing the rlever 44- -to rotate in a clockwise direction about its spherical end 44a. This in turn cause-s the push rod 42 to lbe moved to the left 4in the ydrawings to move the ball valve 36 of its seating and -to permit hydraulic fluid to enter the cavity 32. The hydraulic yiiuid passes through each -of the bores 41 into the gallery 40 and into the main cavity 32. That fluid which enters the cavity 32 passes through the outlet '35 into the conduit 27 to cause a pressure rise in the cylinder 23 of the pump control to cause the piston 21 to move to the right in the drawings. Additionally, the iluid which enters the gallery 40 passes through the passage 59 into the chamber l58 for it to act on the working end of the piston 56. The piston 56 is thus moved to the right in the drawings in opposition to Ythe spring 62, and will move until such time as there is a force acting on the working end of the piston 56 decreasesl ,until a condition of stable equilibrium is reached in the system, i.e., the force 'acting on the 'working end of the' piston 56 is in equilibrium with the force exerted by the spring 62., and the inlet llow past the ball valve 36 is in equilibrium with the outflow of hydraulic uid through the bleed orifice 67 of the bleed valve 68. Under these conditions a pressure will exist -in the cavity 32 and conduit 27 which is directly proportional to the force exerted on the spring 62 by the plunger 64.

If now the force on the plunger 64 should be increased to move it further to the left in the drawings, the spring, spool, and piston 56 Iwill also be moved to the left in the drawings, as will the lever 44 and -push rod 42, to cause the ball valve to move yfurther of it-s seating, thus resulting in an increase in pressure in the cavity 32 and the chamber 53. The increase in force on the plunger 64 thus results .in an increase in pressure in the cavity 32 and in the chamber 58 and acts on the working face of the piston 56 to move the piston I56 .and spool 48 to the right in the drawings to rest-ore the condition lof equilibrium at the determined higher pressure.

Conversely, should the pressure on the plunger 64 be decreased, the lforce acting on the piston 56 will act to move the spool `48 and lever 44 to the right of the drawings, at which time the push rod 42, and ball valve 36 will be moved in a valve closing direction by the -spring 38, that movement being permitted by the counterclockwise movement of the lever 44.

Thus, it will be seen that the control valve of the invention provides an output pressure which is directly proportional to the pressure applied to the plunger 64. It will also be observed that, due to the mechanical advantage of the lever 44, the ratio between the pressure applied to the plunger 64 and the output pressure of the valve may be rela-tively high. In fact, it is found in practice that an output pressure of several hundred pounds can eectively be controlled by the application of pressure to the plunger 64 in the order of a few ounces. It is also found that the valve of the invention is inherently stable in operation and `does not tend to hunt when large or rapid changes are made to the pressure Iapplied to the plunger 64.

It will also be observed that with the plunger 64 in the position shown in the drawings the pump control is set at maximum output in a forward ow direction. By the application of an increasing pressure to the plunger 64 the output of the pump is caused to vary bet-Ween maximum output in the forward ow direction and maximum output in the reverse flow direction through the transitional intermediate position in which the output of the pump is zero.

It is to be understood that the construction above described with reference t-o the schematic drawing is to be taken by way of example only, and that various other constructions are possible within the framework of the invention defined by the appended claims.

What we claim `as our invention is:

l. In a swash plate pump of the kind including a swash plate which provides an annular cam face for moving a series of pistons axially of their bores as the pistons are tra ersed around the cam face on a circular path, the pistons being biased towards the cam face by springs, the swash plate being journalled for movement about an axis`wnich extends substantially diametrically of the cam face and being biased by said springs of the pistons towards an intermediate position of adjustment about said axis in which the cam face is perpendicular to said circular path and in which the output of the pump is zero; means `for moving the swash plate about said axis in opposite directions yfrom said intermediate position of adjustment against the bias imposed by said springs and the force imposed on the swash plate by the pressure of the fluid being pumped to effect a determined output of the pump, said means including a first piston and cylinder arrangement, means for supplying iiuid at a fixed constant pressure to said first piston and cylinder arrangement, means connecting said rst piston and cylinder arrangement to said swash plate Ifor the swash plate to be moved in a first direction about its axis of movement when -tluid under pressure is supplied to said first piston and cylinder arrangement, a second piston and cylinder arrangement, means for supplying iluid at a selectable constant pressure to said second piston and cylinder arrangement, and means connecting said second piston and cylinder are rangement to said swash plate yfor the swash plate to be moved in a second direction opposite to said rst direction about its axis of movement when fluid under pressure is supplied to said second piston and cylinder arrangement, the working area of said second piston and cylinder arrangement being such that at maximum selectable iiuid pressure it exerts a force on said swash plate in said opposite direction which is greater than the force exerted by the rst piston and cylinder arrangement in said `one direction at said xed constant pressure, the swash plate, when the Aforces exerted by said rst and second piston and -cylinder arrangements are equal, lying in said intermediate -position of adjustment in which the output of the pump is zero, said means -for supp-lying fluid at a selectable constant pressurecomprising a valve body having an internal cavity and having an inlet to said cavity' and an outlet from said cavity, a valve member and a comple mentary valve seat associated with said inlet to control the admission ofiluid under pressure into said cavity, and actuating means for moving the valve member ott its seating to admit uid under pressure to said cavity, said actu ating means including a push rod acting on the valve member and movable for moving the valve memberrelatively to its seating, a lever pivoted at one of its ends for movement in a plane which includes the longitudinal axis of the push rod, the lever acting on one end of the push rod at a Iposition intermediate the ends of the lever7 a control .piston having a working face exposed to the lpressure within the cavity, a connection between the control piston and the other end of said lever for the lever to be moved in a valve closing direction by the control piston on an increase in pressure in said cavity, a spring .acting on the lever and biasing the lever in a valve opening direction, and means for varying the bias exerted `by said spring in said valve opening direction.

2. In a swash plate pump of the kind including a swash plate which provides an annular cam face for moving a series of pistons axially of their bores as the pistons are traversed around the cam face on a circular path, the pistons being biased towards Vthe cam face by springs, the swash plate being journalled for movement about an axis which extends substantially diametrically of the cam -t'ace and being biased by said springs of the pistons towards an intermediate position of adjustment about said axis in which lthe cam face is perpendicular to said circular path and in which the output of lthe pump is zero,

arrangement, means for supplying iiuid at a selectable constant pressure to said second piston and cylinder arrangement, and means connecting said second piston and cylinder arrangement to said swash plate Vfor the swash plate to be moved in said second direction opposite to said rst direction about its axis of movement when lluid under pressure is supplied to said second piston and cylinder arrangement, 4the working area of said second piston and cylinder arrangement being such that at a maximum selectable uid pressure it exerts a force o-n said swash plate in said second direction which is at least equal to the `sum of the force exerted by the rst piston and cylinder arrangement at said iixed constant pressure, of the force imposed by the springs of the pistons, and of the force exerted by said pistons during a pumping stroke in order that the swash platebe moved about its axis to an extreme position of movement in said second direction, the swash plate, when the forces exerted by the rst and sec-ond piston and cylinder arangements are equal, lying in said intermediate position of adjustment in which the output of the pump is zero, said means for supplying fluid at a selectable constant pressure comprising a valve body having an internal cavity and having an inlet to said cavity and an outlet from said cavity, a valve member and a complementary valve seat associated with said inlet to control the admission of fluid under pressure into said cavity, the valve member being positioned relatively to its seating so that it is upstream of the seating, spring means biasing the valve member towards its seating, and actuating means for moving the valve member off its seating to admit iiuid under pressure to said cavity, said actuating means including a push rod'acting on the valve member and movable for moving the valve member relatively to its seating, a lever pivoted at one of its ends -for movement in a plane which includes the longitudinal axis of the push rod, the lever acting on one end of the -push rod at a position intermediate the ends of the lever, a spool at the other end of said lever, the spool having a pair of spaced lands respectively slidably guided in .a pair of aligned bores, the axis of said bores being parallel to the longitudinal axis of the push rod, a pivotal connection between said other end of the lever and the spool at a position intermediate the lands, the mutually presented faces of the lands being exposed to the pressure within the cavity, and the mutually `opposed faces being exposed to atmosphere for the spool to .be pressure balanced, a

means yfor moving the swash plate about said axis in opposite directions from said intermediate position of adjustment against the bias imposed by said springs and the force imposed on the swash plate :by the pressure of the fluid being pumped to efect a ,determined output of the pump, said means including .a first piston and cylinder arrangement, means for supplying fluid at a xed constant pressure to said tirst piston and cylinder arrangement, means connecting said rst piston and cylinder arrangement to said swash plate for the swash plate to be moved in a first direction about its axis of movement when tiuid under pressure is supplied to said i'lrst piston and cylinder arrangement, the working area of the first piston and cylinder `arrangement being such that at said fixed constant pressure it exerts a force on said s-Wash plate in said rst direction which is'at least equal to the sum of the force exerted by said springs in a second direction which is opposite to said first direction and the force exerted by said pistons during a pumping stroke in order that the swash plate be moved to anV extreme position about said axis in said first direction, a second piston and cylinder control piston having a working face exposed to the pressure within the cavity, a connection between the control piston and one end of the spool for the spool and lever to be moved in a valve closing direction by the control piston on an increase in pressure in said cavity, a spring acting on the other end of the spool and biasing the lever in a valve opening direction, and means for varying the bias exerted by said spring in said valve opening direction.

Roter-ences Cited by the Examiner UNiTED STATES PATENTS 2,177,098 10/39 Doe et al. 103-38 2,425,779 8/47 Allen 251-321 2,588,866 3/52 Moon 10S- 162 2,669,071 2/54 St. Clair IS7-505.46 2,753,803 7/56y Lawrence 103-162 2,827,069 3/58 Peterson 137-505.46 X 2,875,976 3/59 Harwood 251-321 2,910,008 10/59 Weisenback 10S-162 2,941,480 6/60 Sadler 10S- 102 2,945,449 7/ 60 Le Febure et al. 10S-162 2,987,074 6/61 Niesmann 137-505.46

LAURENCE V. EFNER, Primary Examiner.

JOSEPH H. BRANSOPL'IR., Examiner. 

1. IN A SWASH PLATE PUMP OF THE KIND INCLUDING A SWASH PLATE WHICH PROVIDES AN ANNULAR CAM FACE FOR MOVING A SERIES OF PISTONS AXIALLY OF THEIR BORES AS THE PISTONS ARE TRAVERSED AROUND THE CAM FACE ON A CIRCULAR PATH, THE PISTONS BEING BIASED TOWARDS THE CAM FACE BY SPRINGS, THE SWASH PLATE BEING JOURNALLED FOR MOVEMENT ABOUT AN AXIS WHICH EXTENDS SUBSTANTIALLY DIAMETRICALLY OF THE CAM FACE AND BEING BIASED BY SAID SPRINGS OF THE PISTONS TOWARD AND INTERMEDIATE POSITION OF ADJUSTMENT ABOUT SAID AXIS IN WHICH THE CAM FACE IS PERPENDICULAR TO SAID CIRCULAR PATH AND IN WHICH THE OUTPUT OF THE PUMP IS ZERO; MEANS FOR MOVING THE SWASH PLATE ABOUT SAID AXIS IN OPPOSITE DIRECTIONS FROM SAID INTERMEDIATE POSITION OF ADJUSTMENT AGAINST THE BIAS IMPOSED BY SAID SPRINGS AND THE FORCE IMPOSED ON THE SWASH PLATE BY THE PRESSURE OF THE FLUID BEING PUMPED TO EFFECT A DETERMINED OUTPUT OF THE PUMP, SAID MEANS INCLUDING A FIRST PISTON AND CYLINDER ARRANGEMENT, MEANS FOR SUPPLYING FLUID AT A FIXED CONSTANT PRESSURE TO SAID FIRST PISTON AND CYLINDER ARRANGEMENT, MEANS CONNECTING SAID FIRST PISTON AND CYLINDER ARRANGEMENT TO SAID SWASH PLATE FOR THE WASH PLATE TO BE MOVED IN A FIRST DIRECTION ABOUT ITS AXIS OF MOVEMENT WHEN FLUID UNDER PRESSURE IS SUPPLIED TO SAID FIRST PISTON AND CYLINDER ARRANGEMENT, A SECOND PISTON AND CYLINDER ARRANGEMENT, MEANS FOR SUPPLYING FLUID AT A SELECTABLE CONSTANT PRESSURE TO SAID SECOND PISTON AND CYLINDER ARRANGEMENT, AND MEANS CONNECTING SAID SECOND PISTON AND CYLINDER ARRANGEMENT TO SAID SWASH PLATE FOR THE SWASH PLAT TO BE MOVED IN A SECOND DIRECTION OPPOSITE TO SAID FIRST DIRECTION ABOUT ITS AXIS MOVEMENT WHEN FLUID UNDER PRESSURE IS SUPPLIED TO SAID SECOND PISTON AND CYLINDER ARRANGEMENT, THE WORKING AREA OF SAID SECOND PISTON AND CYLINDER ARRANGEMENT BEING SUCH THAT AT MAXIMUM SELECTABLE FLUID PRESSURE IT EXERTS A FORCE ON SAID SWASH PLATE IN SAID OPPOSITE DIRECTION WHICH IS GREATER THAN THE FORCE EXERTED BY THE FIRST PISTON AND CYLINDER ARRANGEMENT IN SAID ONE 